1. Field of Invention
The invention relates generally to a heat treating furnace used for the brazing and/or annealing of workpieces. More particularly, the invention relates to a system and method for brazing and/or annealing a workpiece which produces forced convection heat transfer and accurate temperature control and uniformity.
2. Description of Related Art
As is generally well-known in the art of heat-treating, brazing and annealing require a close tolerance on the temperature control in order to achieve a desired temperature profile or to maintain temperature uniformity within a workpiece. Convection heat transfer has been found to provide faster heating or cooling of the workpiece on a more uniform basis. In such convection heat transfer, the hot gases are typically directed at the workpiece by jets or fans, with the fans being the more common apparatus to impart velocity to the gases.
Further, the brazing and annealing processes may also have to be performed in a special or protective-treatment atmosphere, such as nitrogen or an exothermic gas, which creates conditions that prevent oxidation of the workpiece, or imparts other desirable characteristics such as the reduction of oxides. When a combination of rapid heat transfer and a controlled atmosphere has been required, the conventional practice has been to introduce the desired atmosphere into a sealed furnace. Furnaces of the so-called continuous operation type have an inlet and an outlet which are provided with baffles so as to reduce loss of heat to the atmosphere. To protect the atmosphere from contamination due to leakage, these furnaces are heated by gas-fired radiant tube burners or electrically-heated elements such as electrical resistors.
In this manner, such convective heat transfer is attained by the fans or jets being mounted in and extending through the side walls or the roof of the furnace. These controlled-atmosphere furnaces generally perform adequately if the furnace insulation does not react with the atmosphere, permit loss of the atmosphere, or cause product/process contamination by deposition of dust resulting from degradation of the insulation. Further, in cases when purity of the atmosphere is critical, the furnace system must be purged for extended periods of time so as to remove the contamination gases from the voids in the insulation.
In order to prevent the contamination problem as described above, a gas-impervious metallic inner liner has been conventionally used to isolate the workpiece in the controlled-atmosphere form the insulation. An example of a conventional convection braze furnace having a lined atmosphere for brazing aluminum heat exchangers is illustrated and described in U.S. Pat. No. 5,147,083 to Gary A. Halstead et al., issued on Sep. 15, 1992 (hereinafter the '083 patent). However, the use of the inner liner suffers from the disadvantage that it is required to be rigidly positioned with respect to fans, doors, and heating/cooling devices, since these items are mounted and sealed to the furnace outer shell.
Another drawback lies in the fact that the need for rigid mounting of the inner liner imposes stresses thereon as a result of differential thermal expansion and contraction between the inner liner and the furnace casing. While various mechanisms have been used to absorb or counteract the stresses due to the differential expansion and contraction, these stresses eventually lead to destruction of the integrity of the liner due to the temperatures normally encountered in the brazing and annealing processes. This problem becomes even more detrimental because replacement of the liner then becomes necessary, which is a very expensive and labor intensive since the furnace must essentially be completely disassembled in order to remove the existing liner and build a new one in its place.
Moreover, another difficulty results from the necessity of supplying heating or cooling through the radiant tubes which isolates a gaseous media from the furnace atmosphere. This results in low heat transfer rates from these items to the recirculating atmosphere. Also, radiant tubes are temperature limited by the materials of construction that can be used, thereby further reducing heating capacity. Leakage of gases through the radiant tubes and leakage around penetrations are also additional sources of atmospheric contamination.
Muffle-type furnaces have also been used conventionally to overcome the disadvantages of the traditional lined controlled-atmosphere furnaces. However, these muffle-type furnaces are also subject to problems. In particular, since the muffle has the capability of being freely movable with the thermal forces, no suitable way has been developed to equip a muffle-type furnace with recirculating fans for imparting convection heat transfer to the workpiece within the muffle so as to produce increased heating rates as well as high thermal efficiencies.